10,781 research outputs found
Is the solar convection zone in strict thermal wind balance?
Context: The solar rotation profile is conical rather than cylindrical as one
could expect from classical rotating fluid dynamics (e.g. Taylor-Proudman
theorem). Thermal coupling to the tachocline, baroclinic effects and
latitudinal transport of heat have been advocated to explain this peculiar
state of rotation. Aims: To test the validity of thermal wind balance in the
solar convection zone using helioseismic inversions for both the angular
velocity and fluctuations in entropy and temperature. Methods: Entropy and
temperature fluctuations obtained from 3-D hydrodynamical numerical simulations
of the solar convection zone are compared with solar profiles obtained from
helioseismic inversions. Results: The temperature and entropy fluctuations in
3-D numerical simulations have smaller amplitude in the bulk of the solar
convection zone than those found from seismic inversions. Seismic inversion
find variations of temperature from about 1 K at the surface up to 100 K at the
base of the convection zone while in 3-D simulations they are of order 10 K
throughout the convection zone up to 0.96 . In 3-D simulations,
baroclinic effects are found to be important to tilt the isocontours of
away from a cylindrical profile in most of the convection zone helped
by Reynolds and viscous stresses at some locations. By contrast the baroclinic
effect inverted by helioseismology are much larger than what is required to
yield the observed angular velocity profile. Conclusion: The solar convection
does not appear to be in strict thermal wind balance, Reynolds stresses must
play a dominant role in setting not only the equatorial acceleration but also
the observed conical angular velocity profile.Comment: 8 pages, 6 figures (low resolution), Accepted by Astronomy and
Astrophysics - Affiliation: (1) AIM, CEA/DSM-CNRS-Univ. Paris Diderot,
IRFU/SAp, France & (2) LUTH, Observatoire de Paris, CNRS-Univ. Paris Diderot,
France ; (3) Tata Institute of Fundamental Research, India; (4) Centre for
Basic Sciences, University of Mumbai, Indi
Interference features in scanning gate conductance maps of quantum point contacts with disorder
We consider quantum point contacts (QPCs) defined within disordered
two-dimensional electron gases as studied by scanning gate microscopy. We
evaluate the conductance maps in the Landauer approach and wave function
picture of electron transport for samples with both low and high electron
mobility at finite temperatures. We discuss the spatial distribution of the
impurities in the context of the branched electron flow. We reproduce the
surprising temperature stability of the experimental interference fringes far
from the QPC. Next, we discuss -- previously undescribed -- funnel-shaped
features that accompany splitting of the branches visible in previous
experiments. Finally, we study elliptical interference fringes formed by an
interplay of scattering by the point-like impurities and by the scanning probe.
We discuss the details of the elliptical features as functions of the tip
voltage and the temperature, showing that the first interference fringe is very
robust against the thermal widening of the Fermi level. We present a simple
analytical model that allows for extraction of the impurity positions and the
electron gas depletion radius induced by the negatively charged tip of the
atomic force microscope, and apply this model on experimental scanning gate
images showing such elliptical fringes
Study of the heating effect contribution to the nonlinear dielectric response of a supercooled liquid
We present a detailed study of the heating effects in dielectric measurements
carried out on a liquid. Such effects come from the dissipation of the electric
power in the liquid and give a contribution to the nonlinear third harmonics
susceptibility chi_3 which depends on the frequency and temperature. This study
is used to evaluate a possible `spurious' contribution to the recently measured
nonlinear susceptibility of an archetypical glassforming liquid (Glycerol).
Those measurements have been shown to give a direct evaluation of the number of
dynamically correlated molecules temperature dependence close to the glass
transition temperature T_g~190K (Crauste-Thibierge et al., Phys. Rev. Lett
104,165703(2010)). We show that the heating contribution is totally negligible
(i) below 204K at any frequency; (ii) for any temperature at the frequency
where the third harmonics response chi_3 is maximum. Besides, this heating
contribution does not scale as a function of f/f_{\alpha}, with f_{\alpha}(T)
the relaxation frequency of the liquid. In the high frequency range, when
f/f_{\alpha} >= 1, we find that the heating contribution is damped because the
dipoles cannot follow instantaneously the temperature modulation due to the
heating phenomenon. An estimate of the magnitude of this damping is given.Comment: 25 pages, 10 figures, Accepted for publication in Journal of Chemical
Physic
Impingement of Water Droplets on NACA 65A004 Airfoil at 8 deg Angle of Attack
The trajectories of droplets in the air flowing past an NACA 65AO04 airfoil at an angle of attack of 8 deg were determined.. The amount of water in droplet form impinging on the airfoil, the area of droplet impingement, and the rate of droplet impingement per unit area on the airfoil surface were calculated from the trajectories and presented to cover a large range of flight and atmospheric conditions. These impingement characteristics are compared briefly with those previously reported for the same airfoil at an angle of attack of 4 deg
Seismic tests for solar models with tachocline mixing
We have computed accurate 1-D solar models including both a macroscopic
mixing process in the solar tachocline as well as up-to-date microscopic
physical ingredients. Using sound speed and density profiles inferred through
primary inversion of the solar oscillation frequencies coupled with the
equation of thermal equilibrium, we have extracted the temperature and hydrogen
abundance profiles. These inferred quantities place strong constraints on our
theoretical models in terms of the extent and strength of our macroscopic
mixing, on the photospheric heavy elements abundance, on the nuclear reaction
rates such as and and on the efficiency of the microscopic
diffusion. We find a good overall agreement between the seismic Sun and our
models if we introduce a macroscopic mixing in the tachocline and allow for
variation within their uncertainties of the main physical ingredients. From our
study we deduce that the solar hydrogen abundance at the solar age is and that based on the Be photospheric depletion, the
maximum extent of mixing in the tachocline is 5% of the solar radius. The
nuclear reaction rate for the fundamental reaction is found to be
MeV barns, i.e., 1.5% higher than the
present theoretical determination. The predicted solar neutrino fluxes are
discussed in the light of the new SNO/SuperKamiokande results.Comment: 16 pages, 12 figures, A&A in press (1) JILA, University of Colorado,
Boulder, CO 80309-0440, USA, (2) LUTH, Observatoire de Paris-Meudon, 92195
Meudon, France, (3) Tata Institute of Fundamental Research, Homi Bhabha road,
Mumbai 400005, India, (4) Department of Physics, University of Mumbai, Mumbai
400098, Indi
Comparative morphometric analysis of bloodstream and lymph forms of Trypanosoma (T.) brucei brucei grown in vitro and in vivo
The fine structure of bloodstream forms of Trypanosoma brucei cultivated in vitro, and of trypanosomes from lymph and blood of mammalian hosts, was compared morphometrically. The cell volume, quantitative parameters of the mitochondrion and of glycosomes were mainly investigated. A Coulter Channelyzer was used for the first time to measure the mean cell volume of living parasites. In vitro, the monomorphic trypanosomes between the feeder layer cells showed lower values for mitochondrial parameters than the slightly pleomorphic forms from the supernatant medium. Trypanosomes in culture were very similar morphologically to forms from lymph nodes of rats. Despite some morphometric differences between cultivated blood stream forms and those grown in vivo, the similarity of both populations was clear. Both populations, however, differed significantly from stages found in the vector or from procyclic culture form
Charting the TeV Milky Way: H.E.S.S. Galactic plane survey maps, catalog and source populations
Very-high-energy (VHE, E>100 GeV) gamma-rays provide a unique view of the
non-thermal universe, tracing the most violent and energetic phenomena at work
inside our Galaxy and beyond. The latest results of the H.E.S.S. Galactic Plane
Survey (HGPS) undertaken by the High Energy Stereoscopic System (H.E.S.S.), an
array of four imaging atmospheric Cherenkov telescopes located in Namibia, are
described here. The HGPS aims at the detection of cosmic accelerators with
environments suitable for the production of photons at the highest energies and
has led to the discovery of an unexpectedly large and diverse population of
over 60 sources of TeV gamma rays within its current range of l = 250 to 65
degrees in longitude and |b|<3.5 degrees in latitude. The data set of the HGPS
comprises 2800 hours of high-quality data, taken in the years 2004 to 2013. The
sensitivity for the detection of point-like sources, assuming a power-law
spectrum with a spectral index of 2.3 at a statistical significance of 5 sigma,
is now at the level of 2% Crab or better in the core HGPS region. The latest
maps of the inner Galaxy at TeV energies are shown alongside an introduction to
the first H.E.S.S. Galactic Plane Survey catalog. Finally, in addition to an
overview of the H.E.S.S. Galactic source population a few remarkable, recently
discovered sources will be highlighted.Comment: 8 pages, 6 figures, in Proceedings of the 48th Rencontres de Moriond
(2013), La Thuile (Italy
Galaxy Selection and Clustering and Lyman alpha Absorber Identification
The effects of galaxy selection on our ability to constrain the nature of
weak Ly\alpha absorbers at low redshift are explored. Current observations
indicate the existence of a population of gas-rich, low surface brightness
(LSB) galaxies, and these galaxies may have large cross sections for Ly\alpha
absorption. Absorption arising in LSB galaxies may be attributed to HSB
galaxies at larger impact parameters from quasar lines of sight, so that the
observed absorption cross sections of galaxies may seem unreasonably large.
Thus it is not possible to rule out scenarios where LSB galaxies make
substantial contributions to Ly\alpha absorption using direct observations.
Less direct tests, where observational selection effects are taken into account
using simulations, should make it possible to determine the nature of Ly\alpha
absorbers by observing a sample of ~100 galaxies around quasar lines of sight
with well-defined selection criteria. Such tests, which involve comparing
simulated and observed plots of the unidentified absorber fractions and
absorbing galaxy fractions versus impact parameter, can distinguish between
scenarios where absorbers arise in particular galaxies and those where
absorbers arise in gas tracing the large scale galaxy distribution. Care must
be taken to minimize selection effects even when using these tests. Results
from such tests are likely to depend upon the limiting neutral hydrogen column
density. While not enough data are currently available to make a strong
conclusion about the nature of moderately weak absorbers, evidence is seen that
such absorbers arise in gas that is around or between galaxies that are often
not detected in surveys.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journa
Pooling quantum states obtained by indirect measurements
We consider the pooling of quantum states when Alice and Bob both have one
part of a tripartite system and, on the basis of measurements on their
respective parts, each infers a quantum state for the third part S. We denote
the conditioned states which Alice and Bob assign to S by alpha and beta
respectively, while the unconditioned state of S is rho. The state assigned by
an overseer, who has all the data available to Alice and Bob, is omega. The
pooler is told only alpha, beta, and rho. We show that for certain classes of
tripartite states, this information is enough for her to reconstruct omega by
the formula omega \propto alpha rho^{-1} beta. Specifically, we identify two
classes of states for which this pooling formula works: (i) all pure states for
which the rank of rho is equal to the product of the ranks of the states of
Alice's and Bob's subsystems; (ii) all mixtures of tripartite product states
that are mutually orthogonal on S.Comment: Corrected a mistake regarding the scope of our original result. This
version to be published in Phys. Rev. A. 6 pages, 1 figur
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